Field of the Invention
The present invention relates to a method of detecting the position of a wafer.
In an ion implantation apparatus such as that disclosed in EP 604066 wafers are loaded one by one from atmospheric conditions into a vacuum chamber through a loadlock chamber. The loadlock chamber is essentially a chamber between a vacuum chamber and the atmosphere which has an outer valve which is selectively operable to seal the loadlock chamber from the atmosphere and an inner valve which is selectively operable to seal the loadlock chamber from the vacuum chamber.
With the outer valve open and the inner valve closed, a wafer is loaded from a supply of untreated wafers into the loadlock chamber by an atmospheric robot. The outer valve is then closed and the loadlock chamber is evacuated. The inner valve is then opened and the wafer is picked up by a vacuum robot and transported to a processing position. Similarly, to transport a processed wafer out of the vacuum chamber, the outer valve is closed and the inner valve is opened allowing the vacuum robot to replace the treated wafer in the loadlock chamber. The inner valve is then closed and the chamber is vented to atmospheric pressure allowing the outer valve to be opened and the processed wafer to be retrieved by the atmospheric robot.
The handoff process between the two robots and the loadlock is done several times a minute, and must be done with great accuracy to avoid sliding the wafer across the loadlock platen which causes particulate contamination on the back of the wafer. The robots used have a tendency to drift over time, particularly as components wear out making accurate repeatable performance over time difficult to achieve. Also, because the encoders in the robots do not provide absolute position readings, they must be taught the handoff position. This calibration can be time consuming and inaccurate when done manually.
According to the present invention a method of detecting the position of a wafer of a known diameter comprises the steps of emitting light from a first source to a first sensor; emitting light from a second source to a second sensor; introducing a wafer such that it intercepts some of the light from the first source directed to the first sensor and the second source directed to the second sensor; and measuring the intensity of light incident on the two sensors.
The method relies on simple optical sensors which provide an analogue signal, and the position of the wafer can be calculated based upon simple geometry using only the outputs from the two sensors and the diameter of the wafer. The absolute position of the wafer can therefore be determined precisely and with a high degree of repeatability.
The invention also extends to a loadlock for the transportation of semi-conductor wafers from the atmosphere into a vacuum chamber, the loadlock comprising a wafer support position in which a wafer is to be supported, a first light source emitting light towards a first light sensor and a second light source emitting light towards a second sensor, the first and second light sources being positioned such that some of the light emitted by both sources towards their respective sensors is intercepted by a wafer in the wafer support position. The position sensing is done independently from a mechanism for moving the wafer.
This apparatus is capable of carrying out the method as defined above.
The light sources can be any source which produces a collimated beam of light, such as an LED. However, the preferred light source is a class 1 modulated infra-red laser. Such a light source is safe enough that it can be used without any shielding. Further, the sensor can sense the light at the frequency of modulation and is thus able to distinguish light from the light source from ambient light. The sensors can be position sensitive diodes. However, simple photodiodes which produce a signal in proportion to the intensity of light incident on them are preferred as they are cheaper.
As the sensing is done optically, the light sources and sensors can be positioned outside of the loadlock chamber behind appropriately positioned windows. This allows the sensing to be done simply and cheaply and allows for easy maintenance without requiring access to the loadlock.
The method of the present invention can also be used in a method of calibrating and operating an apparatus for processing wafers, the apparatus having a gripper on which a wafer is transported, the gripper being driven by one or more servo motors with position encoders, the method comprising the steps of emitting light from a third source to a third sensor; moving the gripper to a predetermined handoff position at which it intercepts a predetermined proportion of the light directed to the third sensor; measuring the intensity of the light reaching the third sensor and reading from the encoder for the or each servo motor; detecting the position of a wafer on the gripper using the method according to the present invention as described above; and for subsequent wafers driving the or each servo motor to the positioned determined by its encoder, and measuring the intensity of the light at the three sensors to verify that the wafer and gripper are correctly positioned.
This method allows the gripper to be taught the hand-off position very simply as the teaching can be done in the software. Furthermore, this method allows for drift monitoring of the operation of the gripper.
In order to determine the location of the wafer in the direction perpendicular to the plane of the wafer, a further light source may be provided which emits light towards a further sensor in a direction perpendicular to a direction in which the light is emitted from the first and second light sources. In this case, the method further comprises raising or lowering the wafer such that the wafer or gripper on which the wafer is supported intercepts some of the light from the further source directed to the further sensor; and measuring the intensity of the light incident on the fourth detector. This provides a way of determining the elevational position of the wafer.
The further light source and sensor can also be used in the type of loadlock which requires elevational movement of a loadlock platen on which the wafer is supported in order to open the loadlock chamber and provide access to the loadlock chamber for the gripper arm. In this case, the method further comprises raising the loadlock platen to a position at which part of the light from the further light source to the further sensor is intercepted by part of the platen. As both the gripper and the loadlock platen are located using the same optical sensor, they can be located very precisely relatively to one another.
This forms a further aspect of the present invention which, in the broadest sense, can be defined as an apparatus detecting that two members are correctly positioned relatively to one another at a predetermined position, the apparatus comprising a light source, a light sensor onto which light from the light source is projected, and a respective flag provided on each member which produces a characteristic shadow on the light detector when in the predetermined position, the characteristic shadow of the two member being different from one another.
Preferably the apparatus further comprises a loadlock chamber relatively to which the light source and light sensor are fixed and the two members are a loadlock platen and a gripper arm which takes wafers to and from the loadlock platen. The flag on one of the loadlock platen and gripper arm is preferably a through hole and the flag on the other of the loadlock platen and gripper arm is preferably a part of the loadlock platen or gripper arm as the case may be arranged to intercept a predetermined proportion of the light projected through the through hole when the gripper arm and loadlock platen are in the predetermined position.
The present invention further encompasses an apparatus for calibrating and controlling an ion implantation apparatus, the ion implantation apparatus having a gripper on which a wafer is transported, the gripper being driven by one or more servo motors, the apparatus for calibrating and controlling comprising a computer, an encoder on each servo motor for producing a signal relating to the position of the servo motor and providing this information to the computer, a connection between the computer and the drive mechanism of each servo motor for controlling the motion of each servo motor, and three sensors for receiving light respectively from three light sources; the computer having a processor arranged to receive and store values indicative of signals from the encoders and sensors recorded when the gripper is at a predetermined handoff position at which the gripper and wafer intercept a predetermined proportion of the light directed to the three sensors, to subsequently output a signal to the servo motors to drive the servo motors to a position at which the reading from each encoder matches the stored value, and to record the signal from each of the three sensors in this position and compare these values with the stored values.
Preferably the processor is further arranged to determine the drift of the gripper by comparing the stored value of each sensor reading with each measured value. Preferably the processor is further arranged to alert an operator if the amount of drift detected exceeds a predetermined threshold.